This invention relates to internal combustion engine oil, operation oil, gear oil and various mechanical lubricant oil composites.
It is desirable for a lubricant oil for practical purposes that the viscosity will not vary over a wide range of from low to high temperature. Ordinarily, viscosity index (hereinafter referred to as VI) is used to show the viscosity/temperature relationship of a lubricant. It is said that VI for petroleum lubricant produced through economical methods such as a solvent refining method is, at best, above 100 except for those produced through special refining methods described in Japanese patent publication No. 50-16803. An additive is needed to obtain a lubricant oil having a higher VI. The additive for this purpose is a viscosity index improver, and an oil-soluble polymer compound having a molecular weight of 10,000 or more is ordinarily used.
There are various types of viscosity index improvers and among those used most widely at present are, polymethacrylate and an ethylenepropylene copolymer (generally called olefin copolymer or OCP). Since a polymer is generally solid, these are generally viscous oil solutions having a concentration of 10-80% so that they can be solubilized by oil.
The most important application of the present viscosity index improver (hereinafter referred to as VI-I=VI improver) is to produce multi-grade engine oil for internal combustion engines. The various types of VI-I are shown in Table 6. It has been, however, used for various purposes such as, in the field of hydraulic oil, as an additive for hydraulic oil in the airline industry, automatic transmission oil (ATF) and shock absorber oil or, in the field of operational oil, as numerical control (NC) machine tool oil which requires an excellent viscosity-temperature relationship. Further, in the field of gear oil, multi-grade oil is required for the purpose of low-temperature shift performance or oil consumption improvement and products having viscosity grades of 80W-90 or 75W-90 have been used.
In the prior lubricant oil for gears, driving chains etc, the higher the speed becomes the more the lubricant oil tends to move apart from the lubricant surface and also the more the oil film tends to detach and scatter. Also, it is required to increase the viscosity of lubricant oil in order to improve the sealing effect of an engine to increase its compression and to decrease blow-by gas. However, this results in loss of power because of resistance due to viscosity. Moreover, it is difficult to keep the oil film for a long period of time and wearing out due to dry start was inevitable.